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Author Topic: ALPA ADJUSTABLE DB ADAPTERS  (Read 1578 times)

jotloob

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ALPA ADJUSTABLE DB ADAPTERS
« on: November 25, 2009, 09:20:10 am »

I wonder , if anyone has experience with the adjustment procedure for an adjustable ALPA DB adapter .

I understand the shimming method , but would very much like to understand , where to start , and in what direction you add or remove shims .
What are the controls .

Jürgen
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Jürgen

archivue

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ALPA ADJUSTABLE DB ADAPTERS
« Reply #1 on: November 25, 2009, 10:13:56 am »

from the alpa website...
fine tune


Can I fine tune the adjustable adpaters for digital use?
Yes, the adjustable adapters can get shimmed in steps of 1/100 mm. The total adjustability is +/- 0.30 millimeter. We recommend to take pictures first and not to play around with adjustments in the first place. If you need help from us or your dealer please have pictures ready. It is much easier to discuss about results than opinions. Please also note that in digital age the best infinity sharpness should be at infinity. Incontrast to film applications where best practice often was to lay infinity before the far point of the depth of field (DOF). Please be also aware of the fact that DOF with digital devices gets rendered much harsher - the transition between "sharp" and "unsharp" is sometimes not so smooth as with film. Please be also aware that all manufactureres of digital backs deliver the same level of "pre-processed" RAW data. Therefore the same camera/lens combination can deliver different results with different backs. RAW data processing - and therefore the software used - is an essential and sometimes decisive element of the whole digital chain.

Preface: The 35 XL and the 24 XL are very delicate lenses. Due do the extreme construction the optical department of Schneider had to define what the best adjustment for these lenses should be. This is because of the curvature of field (bringing a 3D world to the sensors 2D). The center of the lens has to be set slightly beyond infinity (as the Schneider helical and others cannot go beyond infinity) allowing the best result also at wide open towards the edges. If even higher center sharpness is needed (we hope not only for pixel peeping reasons) the lens can be set via the helical at "real" infinity for these lenses.

We therefore not recommend to calibrate the ALPA adapters with the 24 XL and 35 XL but if really needed with longer focal lengths only.

Our calibration is made for the official focal flange distances of the camera manufacturers (Hasselblad, Mamiya et al). A custom made calibration (on a specific back/sensor position) can only be done to infinity (where targets at "infinity" are at least 20-30 times the focal length of the lens away in meters - for the 72 L = some 1500 - 2500 meters, e.g.) with good far sight, without atmospheric interference (with good contrast objects at different distances and farsight/evaluation not hindered by effects like pollution, hot air over the rooftops, wind etc.). Then picture series  with the lens(es) wide open and at f 8/11, with different shims in the adapter has to be done: Normally with shims in steps of 0.1 mm for a rough estimate and then always divide the adjustments in halves narrowing down the range. Or if available with a precision measurement tool: Removing all shims and then altering the focal flange via travel of the helical and afterwards measuring the needed correction via the resulting travel of the hub.
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wolfnowl

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ALPA ADJUSTABLE DB ADAPTERS
« Reply #2 on: November 25, 2009, 03:00:19 pm »

Mark Dubovoy talks about shimming the P65+ back, here.

Mike.
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